Relative reactivities per hydrogen for $1^{\circ} : 2^{\circ}$ C-H are $1 : 3.8$ for chlorination and $1 : 82$ for bromination of propane. The percentages of 2-halopropane in the monohalogenation mixture are, respectively (chlorination, bromination),

Answer: A

Propane has 6 primary H and 2 secondary H. Chlorination:

2 \times 3.8 = 7.6

against

6

, so 2-chloropropane is

7.6/13.6 \times 100 = 55.9

per cent. Bromination:

2 \times 82 = 164

against

6

, so 2-bromopropane is

164/170 \times 100 = 96.5

per cent. This shows the greater selectivity of bromination.